Core-shell fluoropolymer dispersions

ABSTRACT

A dispersion of non-melt-processible fluoropolymer particles having an SSG of less than about 2.225 in aqueous medium. The fluoropolymer particles comprise a core of high molecular weight polytetrafluoroethylene having an average melt creep viscosity greater than about 1.5×10 10  Pa·s and a shell of lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene. The shell has an average melt creep viscosity greater than about 9×10 9  Pa·s and comprises about 5 to about 30% by weight of the particles. The fluoropolymer in the dispersion of the invention is fibrillating.

FIELD OF THE INVENTION

[0001] This invention relates to dispersions of non-melt-processiblefluoropolymers and coatings formed from the dispersions.

BACKGROUND OF THE INVENTION

[0002] Fluoropolymers are applied to a wide number of substrates inorder to confer release, chemical and heat resistance, corrosionprotection, cleanability, low flammability, and weatherability. Coatingsof polytetrafluoroethylene (PTFE) homopolymers and modified PTFE providethe highest heat stability among the fluoropolymers, but unliketetrafluoroethylene (TFE) copolymers, cannot be melt processed to formfilms and coatings. Therefore other processes have been developed forapplying coatings of PTFE homopolymers and modified PTFE. One suchprocess is dispersion coating which applies the fluoropolymer indispersion form. Dispersion coating processes typically employ suchfluoropolymer dispersions in a more concentrated form than theas-polymerized dispersion. These concentrated dispersions contain asignificant quantity of surfactant, e.g. 6-8 weight percent. Suchdispersion coating processes include the steps of applying concentrateddispersion to a substrate by common techniques such as spraying, rolleror curtain coating; drying the substrate to remove volatile components;and baking the substrate. When baking temperatures are high enough, theprimary dispersion particles fuse and become a coherent mass. Baking athigh temperatures to fuse the particles is often referred to assintering. For a number of dispersion coating applications such ascurtain coating or seriography, a fraction of the concentrateddispersion coating stream is deposited on the substrate requiring theremainder of the stream to be recycled. The recycled fraction needs tobe able to withstand the subsequent multiple pumping and mixingoperations necessary for a continuous process. A dispersion suitable forsuch processing should not easily coagulate when subjected to shearingforces. The resistance of the dispersion to premature coagulation can bemeasured by a parameter known as gel time and is an indication of theshear stability of the dispersion.

[0003] In commercial dispersion coating processes, polymers withmoderate molecular weights have typically been used, i.e., having a meltcreep viscosity of approximately 1.2×10¹⁰ Pa·s. For some applicationssuch as PTFE coated fiberglass cloth, it is desired to use highermolecular weight polymer than has typically been used commercially toobtain improvements in flex life and increased lifetimes in industrialand/or architectural applications. However, if higher molecular weightpolymers are used in a commercial process which subjects the polymer tosignificant shear forces, shear stability is adversely affected.

[0004] Fluoropolymers are known which have particles of a high molecularweight core of fibrillating fluoropolymer and a low molecular weightcore of nonfibrillating polymer. Because of the very low molecularweight, non-fibrillating shell on these polymers, resins powders ofthese polymers are not fibrillating polymers. For example, U.S. Pat. No.5,324,785 (Noda et al.) discloses a dispersion of core-shellfluoropolymer particles in which the core is high molecular weightfibrillating PTFE and the shell is nonfibrillating PTFE. The shell ofthis polymer is disclosed to have a molecular weight of 10,000 to800,000 and a melt viscosity of 10² to 10⁶ poises. The objects of theinvention included providing fine particles or powder and a sinteredPTFE powder which have a high molecular weight and good blending ordispersion properties in a resin, an elastomer, a paint, etc. EuropeanPatent Publication EP 0 758 010 A1 discloses a similar composition foruse as a anti-drip agent for imparting a flammable thermoplastic resinwith anti-drip property. U.S. Pat. No. 5,707,763 (Shimizu et al.)discloses a similar polymer for use as a binder for battery electrodeswith a fibrillating PTFE core and a shell of any of variety ofnonfibrillating fluorine-containing polymers or copolymers.

[0005] While advantageous for use in blending powders of theabove-described resins with other materials, the presence of the verylow molecular weight, non-fibrillating polymer in the shell createsproblems in dispersion coating applications. A high molecular weightPTFE dispersion is needed which confers the benefits of high molecularweight PTFE while providing good shear stability necessary forcontinuous commercial dispersion coating processes.

BRIEF SUMMARY OF THE INVENTION

[0006] The invention provides dispersion of non-melt-processiblefluoropolymer particles having an SSG of less than about 2.225 inaqueous medium. The fluoropolymer particles comprise a core of highmolecular weight polytetrafluoroethylene having an average melt creepviscosity greater than about 1.5×10¹⁰ Pa·s and a shell of lowermolecular weight polytetrafluoroethylene or modifiedpolytetrafluoroethylene. The shell has an average melt creep viscositygreater than about 9×10⁹ Pa·s and comprises about 5 to about 30% byweight of the particles. The fluoropolymer in the dispersion of theinvention is fibrillating.

[0007] Preferably, the average melt creep viscosity of thepolytetrafluoroethylene or modified polytetrafluoroethylene of the shellis at least 0.1×10¹⁰ Pa·s less, more preferably at least 0.2×10¹⁰ Pa·sless, than the average melt creep viscosity of polytetrafluoroethyleneof the core. Most preferably, the shell of lower molecular weightpolytetrafluoroethylene or modified polytetrafluoroethylene has anaverage melt creep viscosity of about 9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.Preferably, the shell is polytetrafluoroethylene.

[0008] The invention further provides a coating composition comprising30 to about 70 weight % of the fluoropolymer particles and a surfactantand substrates coated with the composition. Non-melt-processiblefluoropolymer powder obtained by coagulating and drying the aqueousdispersion is also provided.

[0009] Further provided is a batch process for producingnon-melt-processible fluoropolymer dispersion comprising polymerizingtetrafluoroethylene in an aqueous medium in the presence a dispersingagent to produce fluoropolymer having an SSG of less than 2.225. Thepolymerizing is carried out in a first stage during which a first amountof free radical initiator is added and second stage during which asecond amount free radical initiator and a telogenic agent are added.The first amount of initiator produces polytetrafluoroethylene having anaverage melt creep viscosity greater than about 1.5×10¹⁰ Pa·s, and thesecond amount of initiator is at least about 10 times the first amountand being added before about 95% of the total tetrafluoroethylene hasbeen polymerized. The second amount of initiator producespolytetrafluoroethylene or modified tetrafluoroethylene having anaverage melt creep viscosity greater than about 9×10⁹ Pa·s and less thanthe average melt creep viscosity of the polytetrafluoroethylene of thecore.

[0010] Coating compositions in accordance with the invention possesshigh shear stability with the concentrated dispersions preferably havinga gel time greater that 700 seconds when the fluoropolymer solidscontent is about 60 weight % and the surfactant content is about 6weight %. In applications in which the coating is subjected to flexingas in coatings of glass cloth, the flex life of the coating is improved.Dispersions of this invention have an MIT Flex Life for coated glassfabric of greater than 10,000 cycles in the warp direction and/or in thefill direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a graph showing the average melt creep viscosity (MCV)and the instantaneous melt creep viscosity, both to the 1/3.4 power, ofpolymer formed during the process of this invention with respect topercent batch completion.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The invention relates to a dispersion of non-melt-processiblefluoropolymer particles in aqueous medium. The fluoropolymer particlescomprise a core of high molecular weight polytetrafluoroethylene (PTFE)and a shell of lower molecular weight polytetrafluoroethylene ormodified polytetrafluoroethylene.

[0013] Polytetrafluoroethylene (PTFE) refers to the polymerizedtetrafluoroethylene by itself without any significant comonomer present.Modified PTFE refers to copolymers of TFE with such small concentrationsof comonomer that the melting point of the resultant polymer is notsubstantially reduced below that of PTFE. The concentration of suchcomonomer is preferably less than 1 weight %, more preferably less than0.5 weight %. The modifying comonomer can be, for example,hexafluoropropylene (HFP), perfluoro(methyl vinyl ether) (PMVE),perfluoro(propyl vinyl ether) (PPVE), perfluoro(ethyl vinyl ether)(PEVE), chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene (PFBE),or other monomer that introduces side groups into the molecule.

[0014] The fluoropolymer particles have a standard specific gravity(SSG) of less than 2.225, preferably less than 2.220, and morepreferably from 2.180 to 2.215. The SSG is generally inverselyproportional to the molecular weight of PTFE or modified PTFE. SSGalone, however, cannot specify molecular weight as it is also dependenton the presence of modifier, the amount of modifier, and/or initiationby hydrocarbon initiators such as DSP. Also no agreement exists as tothe correct mathematical form the relationship takes. The firstrepresentation of that relationship is expressed in a paper presented byDoban et al. at an ACS meeting Sep. 18, 1956 which gives the numberaverage molecular weight to be

{overscore (M_(n))}=0.597 [log₁₀(0.157/(2.306−SSG)]⁻¹

[0015] with graphical data given in Sperati & Starkwather, Fortschr.Hochpolym-Forsch. Bd. 2,S.465-495 (1961). Another expression of thisrelationship is stated by Noda et al. in U.S. Pat. No. 5,324,785 as:

_(Log) ₁₀ M _(n)=31.83−11.58×SSG

[0016] in which M_(n) is average molecular weight. These equationsresult in different molecular weights for the same SSG values.

[0017] Molecular weight can be more consistently related to melt creepviscosity (MCV) values for PTFE polymers and melt creep viscosity isused in the present application to describe the molecular weight of thepolymer. Molecular weight is linearly related to melt viscosity in Pa·sto the 1/3.4 power as stated in the following:

{overscore (M_(n))}=(MCV ^(1/3.4)−663.963)/0.00021967

[0018] Melt creep viscosities for the fluoropolymer in accordance withthe invention are preferably greater than about 1.4×10¹⁰ Pa·s, morepreferably greater than about 1.5×10¹⁰ Pa·s. Melt creep viscosity inthis application is measured by the procedure U.S. Pat. No. 3,819,594with certain modifications discussed below.

[0019] The fluoropolymer dispersion of this invention is made bydispersion polymerization (also known as emulsion polymerization). Theproduct of dispersion polymerization can be used as aqueous dispersion,optionally after concentrating and/or stabilizing with added surfactantas known in the art, or can be coagulated, isolated from the liquidmedium, and dried. The concentrated dispersions are useful as coating orimpregnating compositions and to make cast films.

[0020] In the manufacture of dispersions in accordance with theinvention, the polymerization is carried out to form a particlestructure in which molecular weight, and in some embodiments,composition vary from one stage of polymerization to another. Thevariation can be can be envisioned so as to view the particle as havingdiscrete layers. While the properties of the “core” and “shell” cannotbe measured independently by analytical methods, these concepts areequated with polymer formed, respectively, in first and later stages inthe polymerization. The process produces PTFE of high molecular weightat the core of the particle and PTFE or modified PTFE of lower molecularweight near and/or at the surface of the dispersion particles. As willbe discussed below, the distinction made herein between core and shellrelates to the amount of initiator present during the first (core) stageof polymerization and during the later (shell) stage of polymerizationas well as the presence or absence of telogenic agent and comonomerbeing introduced.

[0021] Particularly because of the core shell nature of thefluoropolymers of this invention, the melt creep viscosity measured atthe end of the batch is a weighted average of melt creep viscosities ofthe PTFE formed during the batch. For a growing particle, eachincremental volume with its molecular weight contributes to the average.If, for instance, the molecular weight is increasing during the batch,each incremental volume has a higher molecular weight than the lastincremental volume and the average molecular weight is always lower thanthat of the last volume increment. The molecular weight of a volumeincrement is termed the instantaneous molecular weight and the numberaverage molecular weight is given by the expression$\overset{\_}{M_{n}} = \frac{\lim\limits_{n\rightarrow\infty}{\sum\limits_{i = 1}^{n}{M_{n\quad i}\Delta \quad V}}}{\lim\limits_{n\rightarrow\infty}{\sum\limits_{i = 1}^{n}{\Delta \quad V}}}$

[0022] where M_(ni) is the instantaneous molecular weight and ΔV is avolume or weight increment. The instantaneous molecular weight for eachvolume increment is a value selected such that a numerically integratedsolution of the above expression yields the experimentally determinedaverage molecular weight at any point during the batch.

[0023] For the purposes of the present invention, the average molecularweight Mn_(ni) of the shell is determined by the numerical integration,using at least 5 volume or weight increments beginning with andincluding the increment in which the M_(ni) is the highest andconcluding with the end of the batch. The M_(n) for the core isdetermined similarly using at least 30 volume or weight incrementsbeginning with the start of polymerization and ending with and includingthe increment in which the M_(ni) is the highest. Average melt creepviscosity is then determined using the formula stated above for therelationship of melt creep viscosity to M_(n).

[0024] In accordance with the invention, the core of the particlescomprises high molecular weight polytetrafluoroethylene having anaverage melt creep viscosity greater than about 1.5×10¹⁰ Pa·s. The shellcomprises lower molecular weight polytetrafluoroethylene or modifiedpolytetrafluoroethylene with an average melt creep viscosity greaterthan about 9×10⁹ Pa·s. Preferably, the average melt creep viscosity ofthe polytetrafluoroethylene or modified polytetrafluoroethylene of theshell is at least 0.1×10¹⁰ Pa·s less, more preferably at least 0.2×10¹⁰Pa·s less, than the average melt creep viscosity ofpolytetrafluoroethylene of the core. Most preferably, the shell of lowermolecular weight polytetrafluoroethylene or modifiedpolytetrafluoroethylene has an average melt creep viscosity of about9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.

[0025] In fluoropolymers in accordance with the invention, the shellcomprises about 5 to about 30% by weight of the particles. Preferably,the shell comprises about 5 to about 25% by weight of the particles,most preferably, about 5 to about 20% by weight of the particles.Preferably, the shell of the particles is polytetrafluoroethylene.

[0026] Fluoropolymers in accordance with the invention have the generalcharacter of known PTFE polymers made by dispersion polymerizationprocesses. The resins of this invention isolated from dispersion anddried are non-melt-processible. By non-melt-processible, it is meantthat no melt flow is detected when tested by the standard melt viscositydetermining procedure for melt-processible polymers. This test isaccording to ASTM D-1238-00 modified as follows: The cylinder, orificeand piston tip are made of corrosion resistant alloy, Haynes Stellite19, made by Haynes Stellite Co. The 5.0 g sample is charged to the 9.53mm (0.375 inch) inside diameter cylinder which is maintained at 372° C.Five minutes after the sample is charged to the cylinder, it is extrudedthrough a 2.10 mm (0.0825 inch diameter), 8.00 mm (0.315 inch) longsquare-edge orifice under a load (piston plus weight) of 5000 grams.This corresponds to a shear stress of 44.8 KPa (6.5 pounds per squareinch). No melt extrudate is observed.

[0027] A fluoropolymer in accordance with the invention is fibrillating.Fine powder resin isolated from dispersion and dried can be formed intouseful articles by a lubricated extrusion process known as pasteextrusion. The resin is blended with a lubricant and then shaped by anextrusion process. The beading obtained is coherent and microscopicexamination reveals that many particles are linked by fibrils of PTFEwhich have been formed despite the procedure being conducted well belowthe melt temperature. Thus by “fibrillating”, it is meant that alubricated resin forms a continuous extrudate when extruded through a1600:1 reduction die at 18.4 weight percent isoparaffin lubricant soldunder the trademark Isopar® K by ExxonMobil Chemical. A furtherstrengthening of the beading beyond the “green strength”0 obtained byfibrillation is accomplished by sintering after the lubricant has beenvolatized.

[0028] In preferred dispersions in accordance with the invention, atleast about 1.5 weight % of the fluoropolymer particles comprisesubstantially rod-shaped particles having a length to diameter ratio ofgreater than 5. By rod-shaped it is meant that the particles areelongated such that they have a length to diameter ratio of greater thanfive. Some of the rod-shaped particles appear to be straight and somethe rod-shaped particles appear to be bent. In a preferred embodiment,about 1.5 to about 25 weight % of the fluoropolymer particles consist ofsubstantially rod-shaped particles having a length to diameter ratio ofgreater than 5, more preferably at least about 1.5 to about 20 weight %,and most preferably about 2 to about 20 weight %. In a preferredembodiment of the invention, at least 50% the fluoropolymer particles ofthis invention are generally cylindrical having a length to diameterratio of greater than 1.5. More preferably, at least about 90% of thefluoropolymer particles in the dispersion according to invention aregenerally cylindrical in shape with only a small minority of theparticles being generally spherical. Preferably, The dispersionparticles produced have a number average length of about 220 to about500 nm and a number average diameter of from about 150 to about 300 nm.In a preferred embodiment, the dispersion particles have a numberaverage length of about 250 to about 500 nm and a number averagediameter of about 150 to about 250. The rod-shaped dispersion particleshave a number average diameter of less than about 150 nm. It has beenobserved, as described in the Examples of this invention, that CCTincreases with particle size.

[0029] The dispersions containing rod-shaped particles in amounts inaccordance with preferred forms of the invention provides high CCT tothe dispersion coatings of this invention. As will be exemplified, thepresence of too few rods reduces the CCT. Preferably, the number ofrod-shaped particles does not exceed 25 weight % because the presence oftoo many rod-shaped particles is detrimental to CCT.

[0030] The presence of core-shell particles provides high shearstability of the dispersion coating compositions of this invention. Thehigh shear stability permits these coatings to withstand forces appliedby shear generated by pumping and mixing operations. High shearstability facilitates internal recycling of coatings necessary forcontinuous operations for many application processes.

[0031] The invention provides a concentrated dispersion ofnon-melt-processible fluoropolymer particles in aqueous mediumcontaining a surfactant and about 30 to about 70 weight % fluoropolymer.Preferably, the dispersion has a CCT of greater than 24 micrometers whenthe fluoropolymer solids content is about 60 weight % and the surfactantcontent is about 8 weight %. In another preferred embodiment,concentrated dispersions of this invention are characterized by a CCT ofgreater than 20 micrometers when the fluoropolymer solids content isabout 60 weight % and the surfactant content is about 6 weight %.

[0032] Preferably, the concentrated dispersion with about 60 weight %fluoropolymer and about 6 weight % surfactant has a gel time of greaterthan about 700 seconds, greater than about 800 seconds, and mostpreferably greater than 1000 seconds.

[0033] In accordance with the invention, a batch polymerization processis provided for producing a non-melt-processible dispersion. Thepolymerization process preferably involves the steps of prechargingdeionized water to a stirred autoclave and precharging saturatedhydrocarbon having more than 12 carbon atoms which is liquid underpolymerization conditions (preferably paraffin wax) and a dispersingagent (fluorinated surfactant), preferably a perfluorinated carboxylicacid having 6 to 10 carbon atoms. The hydrocarbon acts as a stabilizerin the polymerization process, preventing or retarding the formation ofcoagulated polymer in the agitated system. The process further involvesdeoxygenating, pressurizing the autoclave with TFE to predeterminedlevel, agitating, and bringing the system to desired temperature, e.g.,60°-100° C.

[0034] To form the core, the polymerization is carried out in a firststage during which a first amount of free radical initiator, andadditional dispersing agent (fluorinated surfactant) are added to theautoclave. The first amount of initiator producespolytetrafluoroethylene having an average melt creep viscosity greaterthan about 1.5×10¹⁰ Pa·s. Preferably, the first amount of initiatorproduces polytetrafluoroethylene having an average melt creep viscosityof greater than about 1.0×10¹⁰ Pa·s before about 30% of the totaltetrafluoroethylene has been polymerized (including thetetrafluoroethylene displaced from the vapor space by the volume ofpolymer grown). During the first stage of the polymerization, theaddition of agents providing telogenic activity is preferably minimizedand most preferably, the first stage is carried out without addingtelogenic agents. In preferred forms of the present invention, theseconditions promote the formation of rod-shaped particles i.e., having alength to diameter ratio greater than about 5. In addition, theseconditions preferably promote the formation of large amounts ofgenerally cylindrical particles having a length to diameter ratiogreater than about 1.5. The polymerization proceeds and additional TFEis added to maintain pressure. Then, during the second stage of thereaction, a second amount of free radical initiator is added with atelogenic agent and, for modified PTFE, a comonomer. The second amountof initiator produces lower molecular weight polytetrafluoroethylene ormodified polytetrafluoroethylene with an average melt creep viscositygreater than about 9×10⁹ Pa·s and less than the average melt creepviscosity of polytetrafluoroethylene of the core. Preferably, theaverage melt creep viscosity of the polytetrafluoroethylene or modifiedpolytetrafluoroethylene of the shell is at least 0.1×10¹⁰ Pa·s less,more preferably at least 0.2×10¹⁰ Pa·s less than the average melt creepviscosity of polytetrafluoroethylene of the core. Most preferably, thepolymer produced for the shell of lower molecular weightpolytetrafluoroethylene or modified polytetrafluoroethylene has anaverage melt creep viscosity of about 9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.The second amount of initiator is at least about 10 times the firstamount of initiator, preferably at least about 25 times the firstamount, more preferably at least about 50 times the first amount, andmost preferably at least about 100 times the first amount. The secondamount of initiator and telogenic agent are added before about 95% ofthe total tetrafluoroethylene are polymerized. The second amount ofinitiator and telogenic agent are preferably added when at least about70% of the total TFE has been polymerized, more preferably at leastabout 75% and most preferably at least about 80%.

[0035] During the first stage of the reaction, a high molecular weightcore of PTFE is formed that is preferably at least about 70% of the massof the fluoropolymer particle, more preferably at least about 75%, andmost preferably at least about 80%. During the second stage of thereaction a shell of low molecular weight PTFE or modified PTFE ispreferably formed that is complimentarily no more than about 30% of themass of the fluoropolymer particle, more preferably no more than about25% and most preferably no more than about 20%.

[0036] When the desired amount of TFE is consumed, the feeds arestopped, the reactor is vented, and the raw dispersion is dischargedfrom the polymerization vessel. The supernatant paraffin wax is removed.The dispersion is coagulated, stabilized or concentrated depending onintended end use.

[0037] A graphic description of the process for an embodiment of thisinvention embodiment is illustrated in FIG. 1. The graph is a plot ofthe melt creep viscosity (MVC) to the 1/3.4 power of a preferreddispersion polymerization process of this invention. The average MCV tothe 1/3.4 power of the growing polymer is plotted against the percentageof total tetrafluoroethylene polymerized. It is to be noted that thepercentages of total TFE consumed is analogous to the fraction ofparticle volume or weight formed.

[0038] As stated earlier, the MCV is can be correlated with themolecular weight of the polymer. Curve A represents the average MCV tothe 1/3.4 power of polymer at various stages in the completion of thebatch polymerization. All references in this application to % completionof batch polymerization include the tetrafluoroethylene displaced fromthe vapor space by the volume of polymer grown. In general the molecularweight of the batch increases until a decline of the curve begins atabout 88% of total polymer formation. The increase of average MCV(increase in molecular weight) illustrates the formation of a highmolecular weight core of PTFE in the first stage of the polymerization.The slight decrease of average MCV (decrease in molecular weight)towards the end of the polymerization is attributable to the formationof the lower molecular shell in the second stage of the reaction. Forthis embodiment the average MCV values of the polymer obtainable fromCurve A indicate an average MCV of about 1.3×10¹⁰ Pa·s at 30%completion; an average MCV of about 2.1×10¹⁰ Pa·s at 88% completion andan average MCV of about 1.8×10¹⁰ Pa·s at 100% completion. The maximumaverage MCV (maximum molecular weight) is obtained at about 88%completion just prior to the addition of telogenic agent and moreinitiator and shell formation. The final average MCV value at 100%completion is indicative of the high molecular weight desired for PTFEdispersions in use in order to achieve high flex life.

[0039] A more vivid illustration is represented by Curve B. Curve B is atheoretical depiction of the “instantaneous MCV” to the 1/3.4 power ofpolymer at various stages in the completion of the batch polymerization.The instantaneous MCV, as defined earlier, shows the effect of thechanging recipe conditions on the volume increment growing on thesurface of a particle at that instant. The instantaneous MCV andassociated instantaneous molecular weight increases until the shellportion of the batch is begun. The precipitous decline of theinstantaneous MCV reflects the addition of telogenic agents and addedinitiator. For this embodiment, the instantaneous MCV values of thepolymer obtainable from Curve B indicate an instantaneous MCV of about2.0×10¹⁰ Pa·s at 30% completion; an instantaneous MCV of about 3.1×10¹⁰Pa·s at 88% completion and an instantaneous MCV of about 6.3×10⁹ Pa·s at100% completion.

[0040] The dispersing agent used in this process is preferably afluorinated surfactant. Preferably, the dispersing agent is aperfluorinated carboxylic acid having 6-10 carbon atoms and is typicallyused in salt form. Suitable dispersing agents are ammoniumperfluorocarboxylates, e.g., ammonium perfluorocaprylate or ammoniumperfluorooctanoate.

[0041] The initiators preferably used in the process of this inventionare free radical initiators. They may be those having a relatively longhalf-life, preferably persulfates, e.g., ammonium persulfate orpotassium persulfate. To shorten the half-life of persulfate initiators,reducing agents such as ammonium bisulfite or sodium metabisulfite, withor without metal catalysis salts such as Fe (III), can be used.

[0042] In addition to the long half-life persulfate initiators preferredfor this invention, small amounts of short chain dicarboxylic acids suchas succinic acid or initiators that produce succinic acid such asdisuccinic acid peroxide (DSP) may be also be added in order to reducecoagulum.

[0043] To produce the high molecular weight PTFE core, preferably notelogenic agent is added in the first stage of the reaction. Inaddition, quantities of agents with telogenic activity are minimized. Incontrast, in the second stage of the reaction, such agents in additionto more initiator are added to reduce the molecular weight of thatreached in the core. For the purposes of this patent application, theterm telogenic agent broadly refers to any agent that will prematurelystop chain growth and includes what is commonly known as chain transferagents. The term chain transfer implies the stopping of growth of onepolymer chain and the initiation of growth of another in that the numberof growing polymer radicals remains the same and the polymerizationproceeds at the same rate without the introduction of more initiator. Atelogenic agent produces lower molecular weight polymer in its presencethan in its absence and the number of polymer chain radicals growingeither remains the same or decreases. In practice most agents, ifpresent in sufficient quantities, tend to decrease the number ofradicals and ultimately the polymerization rate. In order to maintainrate, addition of initiator with or near the time of the agent isdesirable. The telogenic agents used in this invention to produce thelow molecular weight shell are typically non-polar and may includehydrogen or an aliphatic hydrocarbon or halocarbon or alcohol having 1to 20 carbon atoms, usually 1 to 8 carbon atoms, e.g., alkanes such asethane, or chloroform or methanol. Also effective are mercaptans such asdodecylmercaptan.

[0044] In producing a shell of modified PTFE, in addition to telogenicagent, comonomer is added in the second stage of the reaction. As statedabove typical comonomers include hexafluoropropylene (HFP),perfluoro(methyl vinyl ether) (PMVE), perfluoro(propyl vinyl ether)(PPVE), perfluoro(ethyl vinyl ether) (PEVE), chlorotrifluoroethylene(CTFE), and perfluorobutyl ethylene (PFBE).

[0045] Factors in the formation of rod-shaped particles include theamount of initiator present in the reaction as well as the presence orabsence of telogens, comonomers and certain surfactants. High levels ofinitiator suppress the formation of rod-shaped particles and thereforein polymerization the dispersions of this invention low levels ofinitiator are desired. Likewise substances with telogenic activity andcertain comonomers such as PAVE suppress the formation of rod-shapedparticles and therefore their absence is desired in the formation of thepolymer core in the first stage of the reaction. Other agents such asthe dispersing agent must be chosen carefully as they can play abeneficial or deleterious role in increasing the fraction of rod-shapedparticles. For instance, alkyl phenol ethoxylates such as Triton® X-100,available from Dow Chemical, are avoided as dispersing agents in thefirst stage of the polymerization as they suppress the formation ofrod-shaped particles.

[0046] For greater flex life and tensile strength, PTFE coatings of highmolecular weight are desirable. Since the shell of the particle is oflower molecular weight which provides shear stability, the coremolecular weight is preferably as high as practicable. Conditions suitedto producing a high molecular weight core, i.e., lower initiatorconcentration and reagents that are free of telogenic or chain transferactivity, also produce conditions that favor large particle sizes and aselected fraction of rod-shaped particles for obtaining high CCT.

[0047] For dispersion concentration, a nonionic concentrating surfactantis added to raw dispersion and the polymer is held at a temperatureabove the cloud point of the nonionic surfactant. Once concentrated toabout 30 to about 70 weight % fluoropolymer, and preferably about 45 toabout 65 weight % fluoropolymer, the upper clear supernate is removed.Further adjustment of the final solids concentration and surfactant aremade as needed. One patent illustrative of concentrating is U.S. Pat.No. 3,037,953 to Marks and Whipple. For coagulation, such methods ascoagulation by vigorous agitation, optionally with added electrolyte, orby freezing and thawing, can be used.

[0048] A nonionic surfactant commonly used in dispersion concentrationis Triton® X-100, supplied by Dow Chemical. This surfactant is describedas an octyl phenol ethoxylate. However, in order to achieve anenvironmentally clean manufacturing process while still maintainingexcellent coating performance and shear stability, the preferredsurfactant used in this invention comprises an alcohol ethoxylate ormixture of alcohol ethoxylates of the formula:

R(OCH₂CH₂)_(n)OH

[0049] wherein R is a branched alkyl, branched alkenyl, cycloalkyl, orcycloalkenyl hydrocarbon group having 8-18 carbon atoms and n is anaverage value of 5 to 18. For example, the ethoxylate of this inventioncan be considered to be prepared from (1) a primary alcohol that iscomprised of a hydrocarbon group selected from branched alkyl, branchedalkenyl, cycloalkyl or cycloalkenyl or (2) a secondary or tertiaryalcohol. In any event, the ethoxylate of this invention does not containan aromatic group. The number of ethylene oxide units in the hydrophilicportion of the molecule may comprise either a broad or narrow monomodaldistribution as typically supplied or a broader or bimodal distributionwhich may be obtained by blending.

[0050] The dispersions exemplified herein have high molecular weight anda high shear stability. The dispersions provide coatings for substratessuch as metal and glass fabric. The dispersions are applied tosubstrates and baked to form a baked layer on the substrate. When bakingtemperatures are high enough, the primary dispersion particles fuse andbecome a coherent mass. Coating compositions of dispersions of thisinvention can be used to coat fibers of glass, ceramic, polymer or metaland fibrous structures such as conveyor belts or architectural fabrics,e.g., tent material. The coatings of this invention when used to coatmetal substrates have great utility in coating cooking utensils such asfrying pans and other cookware as well as bakeware and small electricalhousehold appliances such as grills and irons. Coatings of thisinvention can also be applied to equipment used in the chemicalprocessing industry such as mixers, tanks and conveyors as well as rollsfor printing and copying equipment.

[0051] Alternately the dispersions can be used to impregnate fibers forsealing applications and filtration fabrics. Further the dispersions ofthis invention can be deposited onto a support and subsequently dried,thermally coalesced, and stripped from the support to produceself-supporting films cast from the dispersion. Such cast films aresuitable in lamination processes for covering substrates of metal,plastic, glass, concrete, fabric and wood.

[0052] When the dispersion is coated onto glass fabric and baked, theproduct can give a very high flex life making it desirable forarchitectural applications. The dispersion of this invention preferablyhas an MIT Flex Life for coated glass fabric of greater than 10,000cycles in the warp direction or greater than 10,000 cycles in the filldirection using the test and glass fabric described below. In a morepreferred embodiment, the dispersion of this invention has an MIT FlexLife for coated glass fabric of greater than 10,000 cycles in the warpdirection and an MIT Flex Life for coated glass fabric of greater than10,000 cycles in the fill direction.

[0053] The dispersion attributes provide advantages to end useapplications while maintaining the economy of manufacture. The highshear stability enables processing using techniques such as curtaincoating which allow higher production rates and lower manufacturingcosts. The high molecular weight provides for strong, abrasion resistantPTFE coatings films that show good release characteristics. The economyof production arises from short batch times, the lack of complication ofblending of various dispersions, and the lower inventory costs ofmaintaining less material on hand.

TEST METHODS

[0054] Raw Dispersion Properties:

[0055] Solids content of PTFE raw (as polymerized) dispersion aredetermined gravimetrically by evaporating a weighed aliquot ofdispersion to dryness, and weighing the dried solids. Solids content isstated in weight % based on combined weights of PTFE and water.Alternately solids content can be determined by using a hydrometer todetermine the specific gravity of the dispersion and then by referenceto a table relating specific gravity to solids content. (The table isconstructed from an algebraic expression derived from the density ofwater and density of as polymerized PTFE.) Raw dispersion particle size(RDPS) is measured by photon correlation spectroscopy.

[0056] Surfactant Content:

[0057] The surfactant and solids content of stabilized dispersion aredetermined gravimetrically by evaporating a small weighed aliquot ofdispersion to dryness following in general ASTM D-4441 but using a timeand temperature such that water but not the surfactant is evaporated.This sample is then heated at 380° C. to remove the surfactant andreweighed. Surfactant content is usually stated in weight % based onPTFE solids.

[0058] Resin Properties:

[0059] Standard specific gravity (SSG) of PTFE fine powder resin ismeasured by the method of ASTM D-4895. If a surfactant is present, itcan be removed by the extraction procedure in ASTM-D-4441 prior todetermining SSG by ASTM D-4895.

[0060] Melt creep viscosity (MCV) is measured at 380° C. by amodification of the tensile creep method disclosed in U.S. Pat. No.3,819,594, with the mold at room temperature, using a molding pressureof 200 kg/cm² (19.6 MPa), with the molding pressure held for 2 min,using a load (total weight suspended from the sample sliver) that varieswith the MV to obtain a creep rate suitable for measurement, and waitingat least 30 min after application of the load for elastic response to becomplete before selecting viscous response (creep) data for use in thecalculation.

[0061] Rheometer pressure (extrusion pressure) is measured by the methodof ASTM D-4895 Section 10.8 using 18.4 weight % isoparaffin lubricantsold under the trademark Isopar® K (ExxonMobil Chemicals) and a die with1600:1 reduction ratio.

[0062] Beading quality is determined as in U.S. Pat. No. 3,142,665. Thequality of the extrudate is visually rated as “F” for discontinuous andfrom 0 to 10 for continuous extrudate, i.e., from very poor to excellentregularity and freedom from flaws.

[0063] Copolymer Composition:

[0064] Comonomer content of the modified PTFE resins is determined byFourier transform infrared spectroscopy using the method disclosed inU.S. Pat. No. 4,837,267. For PPVE-modified PTFE, a multiplicative factorof 0.97 derived from the calibration curve is used to convert the ratioof the absorbance at 995 cm⁻¹ to that at 2365 cm⁻¹ to PPVE content inweight %.

[0065] Thermal Concentration Procedure:

[0066] In order to determine both critical cracking thickness and shearstability, the raw dispersion as polymerized (approximately 45 % solidsin the examples of this invention) is concentrated. The specific gravityof the raw dispersion is measured. From the relationship betweenspecific gravity and % solids, the weight of water and PTFE solids inone kilogram of dispersion is calculated. 1.0 milliliter of a 10% byweight solution of citric acid in water is added to one kilogram of theraw dispersion. The dispersion is gently stirred to mix the citric acid.Next 5 milliliters of concentrated ammonium hydroxide (28%) is addedfollowed by stirring. A non-ionic surfactant is then added. Thenon-ionic surfactant used in the examples of this invention is Triton®X-100, supplied by Dow Chemical, unless otherwise specified. Thissurfactant is described as an octyl phenol ethoxylate. The neatsurfactant is a liquid at room temperature and has a viscosity of 240cps. The amount used is 6.0% or 8.0% based on the weight on the waterpresent in the one kilogram sample. The dispersion is heated toapproximately 40° C. and then stirred to disperse the surfactant. Thedispersion is covered with a piece of aluminum foil and heated to 80° C.The dispersion is allowed to remain at 80° C. until concentration iscomplete, usually one hour. The dispersion is allowed to cool to roomtemperature and the upper clear supernate phase is removed with a wateraspirator. The concentrated lower phase is stirred and the % solids and% surfactant determined by the methods described above. The % solids and% surfactant are then adjusted to the desired levels, usually 60 %solids and 6.0% or 8.0% surfactant based on the weight of the PTFEsolids.

[0067] Critical Cracking Test Procedure (CCT):

[0068] The CCT test procedure used in the examples is a procedure totest the maximum film thickness that is obtained by coating a PTFEdispersion on a glass substrate. Dispersions are applied to plates byusing Meyer rods and Fixed Path applicators, both available from PaulGardner and Company of Pompano Beach Fla.

[0069] Glass plates (8 in×12 in) are individually identified and reusedduring testing. If a pattern of unusual results is detected from aparticular plate it is noted as anomalous. A new plate is conditioned byuse of standard dispersion, Teflon® T-30 from the DuPont Company,Wilmington Del. using a #10 Meyer rod and completing the proceduredescribed below. After a plate has been used, it is scraped with astraight blade razor, rinsed with water, and wiped dry for reuse.

[0070] To determine the final CCT, an approximation of the CCT range isused. That approximation may be based on prior knowledge or determinedpreliminarily by use of fixed path applicators of 1, 2, 3, and 4 mils.

[0071] The following table gives the average finished film weightobtained by coating a plate with Meyer Rods and Fixed Path Applicators(both purchased from Paul Gardner and Company) using 60 weight % solidsstabilized dispersion as a standard. TABLE 1 Applicator Film Fixed PathWeight Average Meyer Rod (mil) (mg/inch²) #8 7.41 1 8.1 #10 10.5 #12 13#14 14.2 #16 16 2 16.6 #18 17.8 #20 19.7 3 24.5 4 33.4 #24 35.7 5 43.7#30 46 6 54

[0072] From the approximate range known or determined for the dispersionfilms, filtered dispersion (using a 5 micron syringe filter or othersimilar sized filter) is applied to identified glass plates using theappropriate Meyer Rods and Fixed Path applicators. Two passes ofdispersion are applied horizontally across the top of the plate,approximately 2-3 inches from the top, and drawn down with either theMeyer rod or the Fixed Path applicator at as constant a rate aspossible, approximately 1.5-2 seconds/plate. The plates are dried for 4minutes at 100° C. to remove water and baked for 9 minutes at 380° C. tosintering. The plates are removed from the oven and allowed to standuntil they reach room temperature. After cooling, each plate in thesequence of thinner to thicker is held up to a light and examined forfive seconds to determine the presence of cracks. The first cracked filmis identified. A cracked film resembles tiny whiskers. Using a straightblade razor and a template, 2 one square inch specimens are scribed fromeach of the last uncracked and first cracked film to produce 4 e 4 testspecimens. Using a tweezers, the specimens are transferred to ananalytical balance and weighted. The weight of the 4 test specimens isaveraged and multiplied by a factor of 0.737 to give the final CCT inmicrometers.

[0073] The derivation of 0.737 micrometers/g factor is shown below bysolving for thickness.

[0074] Volume=L×W×T

[0075] T=CCT

[0076] V=# mg of film measured/specific gravity of film

[0077] Spec. gravity assumed=2.16 g/mL=2.16×10⁻⁹ mg/cubic micrometers

[0078] L=2.54×10⁴ micrometers

[0079] W=2.54×10⁴ micrometers

[0080] Shear Stability:

[0081] The shear stability of concentrated dispersions is determined bya gel time test. The dispersion is concentrated as described above and200 ml of dispersion is placed in a Waring commercial explosionresistant blender (Model 707SB, one quart size, 2 speed, airrequirements—10 scfm @ 10 psi, available from Waring of New Hartford,Conn.) and stirred at the highest speed until the dispersion gels. Thegel time is recorded in seconds. The test is terminated after 30 minutesif the dispersion does not gel. Between gel time tests the blender isdisassembled and thoroughly cleaned.

[0082] Particle Morphology and Distribution:

[0083] Dispersion images are obtained with a Hitachi S-4700 fieldemission scanning electron microscope at 700 volts accelerating voltage.Samples are prepared by 1000 fold aqueous dilution of dispersion. Onedrop of diluted dispersion is placed on a piece of polished siliconwafer, water evaporated, and then slowly coated with 2 nanometers ofiridium over a 15 minute period using VCRGroup IBS/TM2005 Ion BeamSputterer. Images are visually inspected and particles are hand counted.Counted particles are modeled as cylinders whose height is the long axisand whose diameter is the short axis. Particle dimensions are measuredwith a ruler in mm and converted to nm using the scale indicated in theSEM image.

[0084] MIT Flex Life:

[0085] The test method is a modification of ASTM D-2176, the standardtest method for folding endurance of paper by the MIT tester. The testis adapted to determine the flex life of dispersion coated glass fabric.The test shows the ability of a fabric to withstand repeated bending,folding and creasing. Dispersions are coated onto a roll of plain wovenbase fabric of glass yarn: warp—EC3 408 tex (34×4×3), 8.1 ends/cm;fill—EC3 408 tex (34×4×3), 7.5 ends/cm. The fabric has a weight of about650 g/m³ and a thickness of about 0.027 inch (0.68 mm). (In thedesignation EC3, E indicates electrical grade glass and C3 indicatescontinuous filament yarn having a filament diameter of 3 micrometers.)The base fabric is available from Fibertech Company located JubailIndustrial City, Saudi Arabia as C1028 raw fabric. The fabric an also beobtained from Saint Gobain located in Merrimack, N.H. The base fabric isimmersion coated to a coating weight of 1500 g/m². The coated fabric isdried at a temperature of 200° F. (93° C.) and then baked to sinteringat a temperature of 750° F. (399° C.).

[0086] Samples of coated glass fabric (½ inch×5 inch, 1.27 cm×12.5 cm)are tested using the standard MIT folding endurance tester described inASTM D-2176 with modifications as herein described. The standard MITFlex Tester, folding endurance test apparatus is available from TiniusOlsen, Testing Machine Co. of Willow Grove Pa. The test apparatus isprovided with a # 8 spring and a 5 lb. load is applied to the spring.

[0087] Tests are conducted in both the warp direction (machinedirection) of the fabric and in the fill direction (cross machinedirection) of the fabric. The results of nine tests are averaged. Thetest results indicate the number of double folds required to break asample piece of fabric.

[0088] Peel Strength:

[0089] The strength of the bond is measured in both the machinedirection and the cross-machine direction between two substrates ofdispersion coated glass fabric having an interlayer of Teflon® FEP filmfused together by heat and pressure. The bonded laminate, when cool, ispulled apart at a 180° peel using a tensile Testing Machine, ConstantRate of Extension, Instron Model No. 4532 (Instron Corporation, Canton,Mass.) operated at 50±3 mm/min (2.0±0.1 in./min) as described inASTM-D4851 (Adhesion of Coating to Fabric Test). The test results arereported as 0.2N/cm (0.1 lb/in. of width). The first inch of bondedsample separation recorded is disregarded. The peel strength is theaverage of the 5 highest and the 5 lowest peaks in the next 3 inches ofsample separation.

EXAMPLES

[0090] Unless otherwise specified, solution concentrations are stated inweight % based on combined weights of solute and solvent water.

Example 1

[0091] This Example illustrates the polymerization of TFE to makefluoropolymer particles of this invention having a high molecular weightcore of PTFE with a low molecular weight shell of PTFE. A polykettlehaving a horizontal agitator and a water capacity of 240 parts by weightis charged with 123.5 parts of demineralized water and 5.82 parts of aparaffin wax supplied by Exxon. Into the evacuated polykettle arecharged, 3.24 parts of a solution containing 0.0616 parts of ammoniumperfluorooctanoate. The contents of the polykettle are agitated at 50rpm. The temperature is increased to 90 C. TFE is then added until thepressure is 2.72 MPa. Then 1.29 parts of a fresh initiator solution of0.01 parts of disuccinyl peroxide (DSP) and 0.00005 parts of ammoniumpersulfate (APS) per part of water are added at the rate of 0.129part/min. Once the pressure has declined by 0.1 MPa, the batch isconsidered to have kicked off. TFE is added at a rate sufficient tomaintain the pressure at 2.72 MPa. Once 8.81 parts of TFE have reactedfrom the kick off, 6.47 aparts of a 2.46 weight % of ammoniumperfluorooctonate solution is added at a rate of 0.324 parts per minute.After 88.1 parts of TFE have been added following initial pressurizingwith TFE, an additional 3.24 parts of a solution of 0.005 parts of APSand 0.06 parts methanol per part of water. Is added at the rate of 0.647part/min. After 96.9 parts of TFE have been added, the TFE feed isstopped and the polykettle pressure is allowed to decrease to 0.79 MPabefore agitation ceases. The time from kickoff to the second initiatoraddition is 68 minutes and the time to the cessation of agitation is 87minutes. Solids content of the raw dispersion is 45.8 weight % and theaverage RDPS is 263 nm. The typical particle shape of the raw dispersionparticles can be described as cylindrical with rounded ends. Only asmall minority of the particles are spherical. Or, as is possible, theparticle is disposed on end so that apparent appearance is spherical. Ahand count of 230 particles of an SEM image of samples obtained from theraw dispersion show the distribution of long and short axis. Someparticles have a small axis of less than 100 nm but a long axis 5 to 20times as large and can be described as rod-shaped. Those particles whoseratio of axes is greater than 5 comprise 10% by number of the particlescounted. If the counted particles are modeled as cylinders whose heightis the long axis and whose diameter is the short axis the weight percentof these particle is 2.8%. By hand measurement, the rod-shaped particleshave average dimensions of 900 nm of length and 68 nm in diameter. Theaverage length of all particles is 413 nm and the average diameter is183 nm. The average melt creep viscosity of the core of the resinparticles is 2.13×10¹⁰ Pa·s and the average melt creep viscosity of theshell of the resin particles is 9.3×10⁹ Pa·s. The core comprises 88.3%by weight of the particles, the shell comprising 11.7% by weight. ThePTFE resin obtained has an SSG of 2.1917 and a melt creep viscosity of19.5×10⁹ Pa·s. The concentrated dispersion of 6 weight % Triton® has aCCT of 29.1 micrometers and a gel time of 991 seconds. At 8 weight %Triton® the CCT is improved to 42.5 micrometers.

Example 2

[0092] The same procedure and essentially identical amounts ofingredients are used as in Example 1 except that the concentration ofthe ammonium perfluorooctanoate originally charged is increased to 0.1part per part of water and the pumped ammonium perfluorooctanoatesolution is reduced in concentration to 1 weight %. The solids contentof the raw dispersion is found to be 45.5% and the RDPS is found to be268 nm. A hand measurement of 328 particles from a SEM image shows pillsand rod-shaped particles of average length of 325 and diameter 195. Theparticles whose length is greater than 5 times the diameter have anaverage length of 770 nm and diameter of 78 nm. The fraction ofrod-shaped particles comprises 4.9% and by weight assuming cylindricalgeometry is 1.9%. The PTFE resin obtained has an SSG of 2.2217. The dryresin is a free flowing powder. The resin is paste extruded at areduction ratio of 1600:1 to give a continuous extrudate and anextrusion pressure of 29.4 MPa. The beading quality is visually rated as2. The concentrated dispersion containing 6 weight % Triton® has a CCTof 23.9 micrometers and a gel time of greater than 1800 seconds. At 8weight % Triton® the CCT improves to 42.3 micrometers.

Example 3

[0093] The same procedures and essentially identical amounts ofingredients are used as in Example 2 except that the concentration ofthe ammonium perfluorooctanoate originally charged is decreased to 0.08parts per part of water and the pumped ammonium perfluorooctanoate isincreased in concentration to 2 weight %. The solids content of the rawdispersion is found to be 45.3% and the RDPS is found to be 283 nm. ThePTFE resin obtained has an SSG of 2.209. The concentrated dispersioncontaining 6 weight % Triton® has a CCT of 22.9 micrometers and a geltime of greater than 1800 seconds. At 8 weight % Triton® the CCTimproves to 34.5 micrometers.

Example 4

[0094] This Example illustrates the polymerization of TFE to makefluoropolymer particles of this invention having a high molecular weightcore of PTFE with a low molecular weight shell of modified PTFE.Dispersions with high shear stability and good CCT's are produced. Thesame procedures and essentially identical amounts of ingredients areused as in Example 1 except that at the same time, in addition to thesecond initiator, 0.097 parts of PPVE are added at a rate of 0.097parts/min. The solids content of the raw dispersion is found to be 45.5weight % and the RDPS is found to be 260 nm. A hand count of 92particles of an FIG. 5 a SEM image finds 5.4% to be rod-shaped particleswhich comprise 14.6 weight % by weight of the sample using cylindricalgeometry. The average length of the rod-shaped particles is 1242 nm andthe diameter 125 nm. The average length of all particles is 308 nm anddiameter of 177 nm. The PTFE resin obtained has a SSG of 2.1889, a meltcreep viscosity of 14.0×10⁹ Pa·s and a PPVE content of 0.025 weight %PPVE. The concentrated dispersion containing 6 weight % Triton® has aCCT of 21.6 micrometers and a gel time of 1380 seconds. The CCT at 8weight % Triton® is found to be 21 micrometers.

Example 5

[0095] This Example illustrates fluoropolymer particles having a highmolecular weight core of PTFE with a low molecular weight shell of PTFE.concentrated in a preferred surfactant of an alcohol ethoxylate madefrom a branched primary alcohol surfactant. A dispersion is preparedaccording to the procedure of Example 1 with the exception that the rawdispersion is thermally concentrated using Novel II TDA 9.4 availablefrom Condea Vista Corporation. The neat surfactant is a liquid at roomtemperature and has a viscosity of 100 cps. After adjusting the solidsto 60% and the surfactant to 6% based on PTFE solids, a gel time of 1597seconds is measured.

Example 6

[0096] This Example illustrates the performance properties of thedispersions of this invention when used for coatings and laminatestructures in architectural applications. The example illustrates thatthe dispersions of this invention have outstanding performanceproperties associated with high molecular weight PTFE dispersions whencompared to commercially available PTFE dispersion.

[0097] A core/shell dispersion is prepared according to the proceduredescribed in Example 1 with the exception that twice the amount ofdisuccinyl peroxide (DSP) specified in Example 1 is used. A seconddispersion is prepared using commercially available PTFE resindispersion, Teflono 30 available from E. I. du Pont de Nemours andCompany of Wilmington, Delaware containing 60% PTFE solids and 6% TritonX-100 (Dow Chemical) based on PTFE solids.

[0098] MIT Flex Life

[0099] Each of the dispersions is used to coat glass fabric for the MITFlex Life Test described above. Results are reported in Table 2. TABLE 2MIT Flex Life Dispersion Warp Direction (cycles) Fill Direction (cycles)Core/shell 17,200 21,400 Teflon ® 30 6,700 7,230

[0100] Peel Strength Test—Two rectangular pieces of coated fabric asprepared for the MIT Flex Life test (7×8 inches±⅛ inch) (17.8 cm×20.3cm±0.32 cm) each with the 8 inch (20.3 cm) dimension parallel to thewarp direction (machine direction) are fused together face to back usingTeflon® FEP film (8¼×7¼±⅛ inch, 0.005-inch thick) (21 cm×18.4 cm±0.32cm, 969 0.013 cm thick). Teflon® FEP is available from E. I. du Pont deNemours and Company of Wilmington, Del. The two pieces of fabric arepressed together at 360° C. for 5 minutes at 245 psi (1.69 MPa). Thebonded laminate, when cool, is pulled apart at a 180° peel using aconstant rate of extension testing machine as described above. Resultsreported in Table 3 are the average of five laminate samples. TABLE 3Peel Strength Dispersion lbf/inch (N/cm) Core/shell 25.4 (44.5) Teflon ®30 21.2 (37.1)

What is claimed is:
 1. A dispersion comprising non-melt-processiblefluoropolymer particles having an SSG of less than about 2.225 inaqueous medium, said fluoropolymer particles comprising a core of highmolecular weight polytetrafluoroethylene having an average melt creepviscosity greater than about 1.5×10¹⁰ Pa·s and a shell of lowermolecular weight polytetrafluoroethylene or modifiedpolytetrafluoroethylene, said shell having an average melt creepviscosity greater than about 9×10⁹ Pa·s and comprising about 5 to about30% by weight of said particles, said fluoropolymer being fibrillating.2. The dispersion of claim 1 wherein the average melt creep viscosity ofthe polytetrafluoroethylene or modified polytetrafluoroethylene of saidshell is at least 0.1×10¹⁰ Pa·s less than the average melt creepviscosity of polytetrafluoroethylene of said core.
 3. The dispersion ofclaim 1 wherein the average melt creep viscosity of thepolytetrafluoroethylene or modified polytetrafluoroethylene of saidshell is at least 0.2×10¹⁰ Pa·s less than the average melt creepviscosity of polytetrafluoroethylene of said core.
 4. The dispersion ofclaim 1 wherein said shell of lower molecular weightpolytetrafluoroethylene or modified polytetrafluoroethylene has anaverage melt creep viscosity of about 9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.5. The dispersion of claim 1 wherein said shell of said particles ispolytetrafluoroethylene.
 6. The dispersion of claim 1 wherein saidparticles have a number average particle size of about 250 to about 300nm.
 7. The dispersion of claim 1 having a gel time greater than about700 seconds at about 60 weight % fluoropolymer and at about 6 weight %surfactant.
 8. The dispersion of claim 1 wherein said fluoropolymer hasa melt creep viscosity greater than about 1.4×10¹⁰ Pa·s.
 9. Thedispersion of claim 1 wherein said shell comprises about 5 to about 25%by weight of said particles.
 10. The dispersion of claim 1 wherein saidshell comprises about 5 to about 20% by weight of said particles. 11.The dispersion of claim 1 having an MIT Flex Life for coated glassfabric of greater than 10,000 cycles in the warp direction.
 12. Thedispersion of claim 1 having an MIT Flex Life for coated glass fabric ofgreater than 10,000 cycles in the fill direction.
 13. The dispersion ofclaim 1 having an MIT Flex Life for coated glass fabric of greater than10,000 cycles in the warp and an MIT Flex Life for coated glass fabricof greater than 10,000 cycles in the fill direction.
 14. Anon-melt-processible fluoropolymer powder obtained by coagulating anddrying the aqueous dispersion of claim
 1. 15. A coating compositioncomprising a concentrated dispersion of non-melt-processiblefluoropolymer particles having an SSG of less than 2.225 in aqueousmedium containing a surfactant, said dispersion containing about 30 toabout 70 weight % fluoropolymer, said fluoropolymer particles comprisinga core of fibrillatable high molecular weight polytetrafluoroethylenehaving an average melt creep viscosity greater than about 1.5×10¹⁰ Pa·sand a shell of lower molecular weight fibrillatablepolytetrafluoroethylene or modified polytetrafluoroethylene, said shellhaving an average melt creep viscosity greater than about 9×10⁹ Pa·s andcomprising about 5 to about 30% by weight of said particles, saidfluoropolymer being fibrillating.
 16. The coating composition of claim15 wherein the average melt creep viscosity of thepolytetrafluoroethylene or modified polytetrafluoroethylene of saidshell is at least 0.1×10¹⁰ Pa·s less than the average melt creepviscosity of polytetrafluoroethylene of said core.
 17. The coatingcomposition of claim 15 wherein the average melt creep viscosity of thepolytetrafluoroethylene or modified polytetrafluoroethylene of saidshell is at least 0.2×10¹⁰ Pa·s less than the average melt creepviscosity of polytetrafluoroethylene of said core.
 18. The coatingcomposition of claim 15wherein said shell of lower molecular weightpolytetrafluoroethylene or modified polytetrafluoroethylene has anaverage melt creep viscosity of about 9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.19. The coating composition of claim 15 wherein said shell of saidparticles is polytetrafluoroethylene.
 20. The coating composition ofclaim 15 wherein said particles have a number average particle size ofabout 250 to about 300 nm.
 21. The coating composition of claim 15having a gel time of greater than about 700 seconds at about 60 weight %fluoropolymer and about 6 weight % surfactant.
 22. The coatingcomposition of claim 15 wherein said fluoropolymer particles have a meltcreep viscosity greater than about 1.4 ×10 ¹⁰ Pa·s.
 23. The coatingcomposition of claim 15 wherein said shell comprises about 5 to about25% by weight of said particles.
 24. The coating composition of claim 15wherein said shell comprises about 5 to about 20% by weight of saidparticles.
 25. The coating composition of claim 15 in the form of abaked layer.
 26. A substrate coated with the composition of claim 25.27. The substrate of claim 26 wherein the substrate is metal.
 28. Thesubstrate of claim 26 wherein the substrate is glass fabric.
 29. Aself-supporting film cast from the dispersion of claim
 1. 30. A batchprocess for producing non-melt-processible fluoropolymer dispersioncomprising polymerizing tetrafluoroethylene in an aqueous medium in thepresence a dispersing agent to produce fluoropolymer having an SSG ofless than 2.225, said polymerizing being carried out in first stageduring which a first amount of free radical initiator is added andsecond stage during which a second amount free radical initiator and atelogenic agent are added, said first amount of initiator producingpolytetrafluoroethylene having an average melt creep viscosity greaterthan about 1.5×10¹⁰ Pa·s, and said second amount of initiator being atleast about 10 times said first amount and being added before about 95%of the total tetrafluoroethylene has been polymerized, said secondamount of initiator producing polytetrafluoroethylene or modifiedpolytetrafluoroethylene having an average melt creep viscosity greaterthan about 9×10⁹ Pa·s and less than the average melt creep viscosity ofthe polytetrafluoroethylene of said core.
 31. The process of claim 30wherein said second amount of initiator produces polytetrafluoroethyleneor modified polytetrafluoroethylene having an average melt creepviscosity at least 0.1 ×10¹⁰ Pa·s less than the average melt creepviscosity of the polytetrafluoroethylene produced during said firststage.
 32. The process of claim 30 wherein said second amount ofinitiator produces polytetrafluoroethylene or modifiedpolytetrafluoroethylene having an average melt creep viscosity at least0.2 ×10¹⁰ Pa·s less than the average melt creep viscosity of thepolytetrafluoroethylene produced during said first stage.
 33. Theprocess of claim 30 wherein said second amount of initiator producespolytetrafluoroethylene or modified polytetrafluoroethylene having anaverage melt creep viscosity of about 9×10⁹ Pa·s to about 1.3×10¹⁰ Pa·s.34. The process of claim 30 wherein said first amount of initiatorproduces polytetrafluoroethylene having an average melt creep viscosityof greater than about 1.0×10¹⁰ Pa·s before about 30% of the totaltetrafluoroethylene has been polymerized.
 35. The process of claim 30wherein said fluoropolymer produced is fibrillating.
 36. The process ofclaim 30 wherein polymerizing in said second stage producespolytetrafluoroethylene.
 37. The process of claim 30 wherein said secondamount of initiator and said telogenic agent are added when at leastabout 70% of the total tetrafluoroethylene has been polymerized.
 38. Theprocess of claim 30 wherein said second amount of initiator and saidtelogenic agent are added when at least about 75% of the totaltetrafluoroethylene has been polymerized.
 39. The process of claim 30wherein said second amount of initiator and said telogenic agent areadded when at least about 80% of the total tetrafluoroethylene has beenpolymerized.
 40. The process of claim 30 wherein said polymerizing insaid first stage is carried out without adding telogenic agent.
 41. Theprocess of claim 30 wherein said dispersing agent comprises aperfluorinated carboxylic acid containing 6 to 10 carbon atoms.
 42. Theprocess of claim 30 wherein said second amount of initiator is at leastabout 25 times the said first amount.
 43. The process of claim 30wherein said second amount of initiator is at least about 50 times thesaid first amount.